1. Field of the Invention
The present invention relates to methods of and apparatuses for inspecting the acceptability of soldering of parts mounted on printed circuit boards (PCBs).
2. Description of Prior Art
Originally, PCBs were visually inspected by operators for defects. Specifically, along with other questions regarding the placement of surface mounted devices on lands and their lead connections, the presence or absence, the amount, the solubility, the short, the inferior conduction, and the like of solder determine the acceptability of a PCB. But the drawbacks due to human error caused by fatigue and different standards of acceptability, not to mention the high cost of and extensive time needed for manual inspection of the increasingly small PCBs, led the industry towards automatic inspection. Unfortunately, problems dealing with cost, effectiveness, and speed of inspection still plague the art. Greatly contributing to the problems is the necessity for a reference board or a means to determine the quality of the PCBs to be inspected. Recent technology intimates that the use of reference parameters may be used to overcome these problems by determining for example the quality of a soldered portion (fillet) of a PCB.
The surface of a fillet has a shape which extends in three dimensions. In order to inspect the shape, it is essential that information on three-dimensional shapes can be detected.
FIG. 5 shows an example of an automatic inspecting apparatus capable of inspecting information on three-dimensional shapes, which projects slit light to a fillet on a PCB 2. By projecting the slit light 1, reflected light of a light cutting line 3 formed on the surface of the PCB 2 including the fillet is imaged by an imaging unit 4. Then, the imaged pattern is examined to detect the three-dimensional shape of the fillet.
However, in this inspecting method, information is only obtained on the shape of the fillet illuminated by the slit light 1. Accordingly, it is difficult to grasp the three-dimensional shapes of the other portions.
To solve this problem, there is provided a method of projecting light in a plurality of directions and at different angles of incidence to the surface of a solid bound by a curved surface to be inspected, imaging its reflective light, and detecting the orientation of the element constituting the curved surface from the respective imaged patterns. This "active sensing method" is one way to detect information on three-dimensional images. More specifically, when a light beam having a constant pattern is projected onto an object, the pattern of the reflected light beam obtained from the object is deformed corresponding to the three-dimensional shape of the object and the deformed pattern enables the estimation of the object's shape.
FIG. 6 is a diagram explaining the principle of this method, showing the positional relation between a detecting system comprising a light projecting unit 5, an imaging unit 6, and a solid bound by a curved surface which is an object to be inspected.
When a light beam 8 is projected from the light projecting unit 5, which is arranged in a given position, to the surface of a solid bound by a curved surface (i.e. a fillet) 7, the reflected light beam 9 is incident on the imaging unit 6 which is placed directly over the solid 7. Thus, the portion of the curved surface 7 illuminated by the light beam 8 is oriented at an angle of i with a horizontal reference surface 10 (i is the angle of incidence). Accordingly, when a plurality of light projecting units 5 are projected on a fillet 7, the respective angles of incidence detected by the imaging unit 6 enable the determination of the nature of the fillet 7.
Furthermore, if the light projecting unit 5 projects the light beam 8 having an incident angle ranging from i-.DELTA.i to i+.DELTA.i, the imaging unit 6 can detect a reflected light beam 9 having a width corresponding to the range. In other words, the imaging unit 6 can detect such light as reflected from curved surfaces having an angle of i-.DELTA.i to i+.DELTA.i with respect to reference surface 10.
If, as in FIG. 7, the light projecting unit 5 comprises a plurality of ring shaped light sources 11, 12 and 13 having different angles of incidence to the solid bound by a curved body 7, then the elements of the curved surface having orientations corresponding to the angles of incidence of light beams 14, 15 and 16 from the sourCes 11 , 12 and 13 can be specifically detected as described above.
The three ring-shaped light sources 11, 12, and 13 having radii of r.sub.m (m=1, 2, and 3) are horizontally arranged in positions at the heights h.sub.m (m=1, 2, and 3) from a reference surface 10. In addition, let i.sub.m (m=1, 2, and 3) be the angles of incidence of the light beams 14, 15, and 16 from the light sources 11, 12, and 13 to the solid bound by a curved surface 7. In this case, the elements of the curved surface respectively having angles of inclination of i.sub.m in the solid bound by a curved surface 7 can be detected by the imaging unit 6. The size of the element of the curved surface is sufficiently smaller than the total optical path length leading from the light sources 11, 12, and 13 and the solid 7 to the imaging unit 6. Consequently, the angle of incidence, that is, the angle of inclination of the element of the curved surface to be detected can be set by the following equation: EQU cos i.sub.m =h.sub.m /(h.sub.m.sup.2 +r.sub.m.sup.2).sup.178
More specifically, the foregoing equation can be used to determine specific heights and radii for light sources such that the reflected light from each source is incident upon the imaging unit when each respective light was reflected off of a predetermined slope from the solid. Thus, the imaging unit 6 will receive incident beam 14 from the top ring-shaped light source 11 when the slope of the solid is slight or zero; when the solid 7 has a flat surface. Similarly, light source 12 will send incident beam 15 to the imaging unit 6 from where the solid 7 has a medium or gentle slope and incident beam 16 from light source 13 will only be received by the imaging unit 6 from places where the solid 7 has a steep slope.
An automatic inspecting system as described above consisting of white light sources and a monochromatic camera has been proposed (Japanese Patent Application Laid Open Publication #61-293657). However, this apparatus requires that the light sources be turned on and off instantaneously and in series in order for the imaging unit to distinguish the light received from its source. Thus, a memory for storing images obtained at different timings of projected light, an arithmetic unit for executing an arithmetic operation taking the images as the same field image, a lightning unit for causing each of the light sources to instantaneously perform a lightning operation, and the like are required.
One way to eliminate the need for this timing hardware is to distinguish the different light sources by color as in the related art submitted to the US PTO (U.S. Ser. No. 439,943). This co-pending application is not prior art, but it is related art that describes an earlier embodiment of this invention. Specifically relating to this invention is the embodiment with only one imaging unit described in FIG. 15 of U.S. Ser. No. 439,943. However, this invention is only designed to measure high-grade PCBs which are more expensive than normal PCBs because of a thin coating of solder pasted over the lands.
Referring now to FIG. 8, the light sources 24 described in U.S. Ser. No. 439,943 are from top to bottom red 28, green 29, and blue 30. First, a reference PCB 21s is placed on a conveyor 27 which is located on a y-axis table 23. This reference PCB 21s contains lands covered by copper and electrically conductive patterns formed on the epoxy resin as well as surface mounted devices. Additionally, the lands are further covered by a thin coating of solder (about 150 to 200 micrometers thick) to improve the adhesiveness between the surface mounted device's soldered lead and the land, while the patterns are covered by a rough surfaced, green colored solder resist for insulation and protection against short circuits that may be caused by stray solder. As the PCB 21s moves along the y-axis table 23, the light sources 24 project light and the reflected incident beams are imaged by an imaging unit 25, which is moving along the x-axis. At this point it should be noted that only smooth surfaces such as fillets are able to reflect distinct separate colors back to the imaging unit 25 because rough surfaces randomly diffuse the rays directed thereto, in effect mixing the colors so the incident relection appears as white light. The reference PCB 21s is used to teach the judging means 26 the size and type of parts and the like. After this first teaching has been recorded in the judging means 26, a second reference PCB 20s (not shown) without surface mounted devices is put through the imaging process to teach the judging means 26 land location information to be extracted in accordance with the data taken from the first teaching. Since the whole surface of the PCB 20s is still flat, as no surface mounted devices have yet been placed thereon, the top ringed-light source 28 will highlight the smooth, solder coated lands as red while the rough solder resist will appear as its natural color in white light which is green.
In a similar manner, the PCB to be inspected 21t is placed on the y-axis table 23, the incident beams from the light sources are imaged by the imaging unit 25 and sent to the judging means 26 for determination. Since only the smooth solder portions are highlighted by the respective colors: red for flat surfaces, green for gentle slopes, and blue for steep slopes; and the rest of the PCB is imaged as it appears in white light, the resulting image produced contains very detailed information on the nature of the soldered fillets as well as the whole PCB.
However, while U.S. Ser. No. 439,943 performs admirably on expensive high-grade PCBs with the thin coating of solder over their lands, the apparatus has problems when dealing with less expensive PCBs that have no such coating. Specifically, flat soldered portions are undistinguishable from the rough copper lands. The imaging unit views both as red. Consequently, lead connections completely lacking solder or insufficiently soldered to the lands cannot be detected.